A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured. A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 10-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
It is well known in the art of lithography that an image of the patterning device projected onto a substrate can be improved by appropriately choosing angles at which the patterning device is illuminated, i.e., by appropriately choosing an angular distribution of radiation illuminating the patterning device. In a lithographic apparatus having a Koehler illumination system, the angular distribution of radiation illuminating the patterning device is determined by a spatial intensity distribution of the illumination beam in a pupil plane of the illumination system. This is because the illumination beam at the pupil plane effectively acts as a secondary or virtual radiation source for producing the illumination beam that is incident on the patterning device. The shape of the spatial intensity distribution of the illumination beam at the pupil plane within the illumination system is commonly referred to as the illumination mode or profile. The pupil plane of the illumination system may also be referred to as the pupil plane of the patterning device.
Illumination beams with certain spatial intensity distributions at the pupil plane improve the processing latitudes when an image of the patterning device is projected onto a substrate. In particular, an illumination beam having a spatial intensity distribution with a dipole, annular or quadrupole off-axis illumination mode can enhance the resolution and/or other parameters of the projection, such as sensitivity to projection system optical aberrations, exposure latitude and depth of focus. Certain “soft-pole” illumination modes may also have an advantageous effect on the image of the patterning device projected onto a substrate. Accordingly, an illumination system typically includes one or more devices or structures to direct, shape and control the illumination beam such that it has a desired spatial intensity distribution/illumination mode at the pupil plane.